A process for producing ethanol, by which lignocellulosic biomass such as rice straw as a substrate is saccharified by a saccharifying enzyme produced by a microorganism and then the yielded saccharide is fermented, has been heretofore known. In this regard, lignocellulosic biomass is constituted such that lignin is bound to cellulose or hemicellulose firmly. Consequently, a presaccharification treatment product is used, which is prepared by pretreating lignocellulosic biomass to dissociate lignin contained in the lignocellulosic biomass or to swell the lignocellulosic biomass, for saccharification.
In this regard, the term “dissociate” means herein at least a part of the bonds between lignin and cellulose or hemicellulose is broken. The term “swell” means that crystalline cellulose expands due to infiltration of a liquid, which generates gaps in cellulose or hemicellulose constituting the crystalline cellulose, or gaps inside a cellulose fiber.
Since a saccharifying enzyme is expensive, it has been common in a conventional ethanol production process that the concentration of a substrate contained in a presaccharification treatment product is set at a low level in order to save the consumption of a saccharifying enzyme. However, if the concentration of a substrate contained in a presaccharification treatment product is set at a low level, the resulted concentration of a saccharified solution to be yielded from such a presaccharification treatment product becomes also low, and as a consequence, the concentration of ethanol to be yielded by fermenting the saccharified solution becomes also low. As a result, there occurs a problem that the time and thermal energy required for distillation increase, when the yielded ethanol is distilled to be concentrated.
To solve the problem, it is conceivable to increase the concentration of a substrate contained in a presaccharification treatment product, and also increase the consumption of a saccharifying enzyme, so as to yield ethanol at a high concentration. In this case, however, the cost increases due to the increase in the consumption of an expensive saccharifying enzyme, the cost of the process for producing ethanol as a whole is required to be reduced.
Streamlining of a presaccharification treatment for lignocellulosic biomass is conceivable as a measure for reducing cost in an ethanol production process.
As a presaccharification treatment process for lignocellulosic biomass, a process by which, for example, lignocellulosic biomass is mixed with liquid pure ammonia, heated, pressurized, and then rapidly depressurized, has been known. By doing so, vaporized ammonia gas expands rapidly, so that the lignocellulosic biomass also expands and lignin is removed physically from the lignocellulosic biomass (see Patent Literature 1).
However, use of pure ammonia requires special devices, such as a high pressure container for storage, and therefore use of ammonia water, which is an aqueous solution of pure ammonia, has been studied.
As a process for using ammonia water, a process, by which lignocellulosic biomass is brought into contact with ozone and then immersed in ammonia water, has been known (see Patent Literature 2). In this case, however, a device for contacting lignocellulosic biomass with ozone is necessary, and a process step therefor is added, and consequently the cost cannot necessarily be reduced.
Further, a process, by which a mixture obtained by mixing lignocellulosic biomass and ammonia water is heated, has been known (see Patent Literature 3). According to a process of Patent Literature 3, if lignocellulosic biomass is dispersed in ammonia water with the concentration of 0.8 to 15% by weight and heated, lignin will be dissociated from lignocellulosic biomass or the lignocellulosic biomass will be swollen.